This study is directed at elucidating properties of the biochemical disposition of the enkephalin opioid pentapeptides. Investigations are concerned with identifying the synthesis of enkephalins for larger molecular weight precursors as well as examining alterations during changes in physiological and pharmacological states. presently considerable efforts have resulted in development of sensitive and specific radioimmunoassays which distinguish the two enkephalins and have permitted measurements under various conditions as well as immunohistochemical localization.